1. Field of the Invention
The present invention relates to FM (Frequency Modulation) demodulators for FM demodulating the FM modulated signals, and more particularly, to an FM demodulator for use in a satellite broadcasting receiver.
2. Description of the Background Art
Many satellite broadcasting receivers utilize an FM demodulator with a PLL (Phase Locked Loop) in order to reduce threshold noise.
FIG. 8 is a block circuit diagram of a conventional FM demodulator. As shown in FIG. 8, an FM modulated signal is input to an input terminal T.sub.in and transmitted through a phase comparator 20, a loop filter 21, a DC (Direct Current) amplifier 22, and a BB (Base Band) amplifier 23, and a demodulated output signal is obtained from an output terminal T.sub.out. The output of DC amplifier 22 is fed back to phase comparator 20 through a VCO (Voltage-Controlled Oscillator).
In the circuit above, phase comparator 20 detects a phase difference between the input FM signal and the output of VCO 24. Loop filter 21 attenuates the noise component of the input signal which falls out of the band. DC amplifier 22 amplifies the.sub.e output of loop filter 21 and outputs the amplified signal to BB amplifier 23 as a demodulated signal. The output of DC amplifier 22 is also fed back to VCO 24 and controls an oscillation frequency thereof.
In a satellite broadcasting receiver, a signal level which is input to the receiver may decline and a CN ratio (Carrier to Noise Ratio) may deteriorate in wet weather, for example, leading to a higher noise level relative to the signal in the FM demodulator. When the noise level exceeds a certain point, impulse noise might be generated sharply to degrade an SN ratio (Signal to Noise Ratio), whereby threshold noise might be generated in a TV screen. Such a symptom is characteristic of FM demodulator.
More specifically, although it is true that FM demodulators have an advantage of improving the SN ratio when the CN ratio is good, they also have a drawback that, when the CN ratio hits a certain point (hereinafter referred to as a threshold level) or lower, noise is sharply increased in the signal in the FM demodulator and the SN ratio is degraded. Generally, the CN ratio and the SN ratio are measured at T.sub.in and T.sub.out, respectively, in FIG. 8.
FIG. 9 shows the characteristics of degradation in the SN ratio of the conventional FM demodulator as described above. The horizontal axis shows the CN ratio and the vertical axis shows the SN ratio. In the figure, a line A shows an ideal characteristic line of the CN ratio versus the SN ratio, and a line B shows a characteristic line of the conventional FM demodulator. A line C is a line used for obtaining a static threshold value of the conventional FM demodulator, in parallel to and lower than line A by 1 dB. The CN ratio at a crossing point P1 of lines B and C is called a static threshold value.
It can be seen from FIG. 9 that a region of the CN ratio where the SN ratio deteriorates becomes lower as the static threshold value as defined above decreases: in other words, the characteristic line becomes closer to line A showing better signal reception characteristics. The static threshold value of the FM demodulator shown in FIG. 9 is the value of the CN ratio at crossing point P1 in the figure, i.e., 5.8 dB.
As can be seen from line B in FIG. 9, the conventional FM demodulator shows nearly ideal characteristics when the CN ratio is approximately 10 dB or higher. However, the SN ratio deteriorates when the CN ratio is approximately 8 dB or lower. Especially, the characteristics of the FM demodulator sharply worsens when the CN ratio is approximately 6 dB or lower.
This gives rise to the following problems. Specifically, although the conventional FM demodulator receives signals almost ideally when the CN ratio is high, that is, in a region or in signal receiving conditions where FM signals reach sufficiently, the reception signal deteriorates and image quality is degraded sharply in a region or in signal receiving conditions where the CN ratio is low, that is, where FM signals do not reach sufficiently.